Molecular-Level Insight into the Hydroxylated Monomeric VO_x/θ-Al₂O₃(010) and Its Adsorption of Methanol

Unraveling the catalytic functionality of hydrophilic surfaces from a molecular level is of a great challenge, but it offers fundamental guidelines for mechanistic understanding of the heterogeneous catalysis and eventually facilitates the development of highly efficient catalysts. Here, we present a combined experimental and theoretical study of hydroxylated monomeric vanadia species deposited on the θ-Al₂O₃(010) surface and the subsequent dissociative adsorption of methanol. First, the structure of a hydroxylated θ-Al₂O₃(010) surface with one monolayer of water coverage is generated. Energies associated with the dissociative adsorption of water indicate that hydroxylation is a strongly exothermic process. The normalized intensities of the vibrational modes of the in situ IR spectra of θ-Al₂O₃ as a function of temperature show that the hydrated θ-Al₂O₃(010) surface gradually dehydrates with increase in temperature up to 300 °C, which agrees well with a previous theoretical calculation and shows the presence of strongly adsorbed surface −OH and water. Second, the hydroxylated monomeric VO_x/θ-Al₂O₃(010) species with tridentate VO_x structures were investigated for the two most stable adsorption geometries, where three V–O–Al interface bonds form between VO_x and the hydroxylated surface. The calculated frequencies of the VO and V–O modes for these two structures agree well with our previous Raman spectra. Finally, dissociative adsorption of methanol for all possible geometries of the two most stable geometries of VO_x/θ-Al₂O₃(010) shows a large range of adsorption energies, with the most exothermic adsorption being 1.29 eV, where the −V–O–CH₃ residue prefers to be dangling toward vacuum. The methoxy group geometries were confirmed by the close matching of the calculated frequencies and measured IR spectra. Overall, our study illuminates details of the lowest energy molecular structures and energies of hydroxylated θ-Al₂O₃(010), VO_x/θ-Al₂O₃(010), and CH₃OH/VO_x/θ-Al₂O₃(010). These species are of great interest because of their importance in reactions involving supported vanadia catalysts, both for interpreting experimental studies, and for future use of the structures determined here in studies of reaction mechanisms.